The Reborn
Another Warband which /tg/ likes to include in the category of "Why the fuck do we even exist?", The Reborn are notable because of their heavily corroded armor and use of very old, almost antiquated weaponry because antique weapons are the most effective in W40K. They were first sighted in 795.M40.
Just like the Dragon Warriors and about half of all other Warbands, they have potential, but due to extreme laziness, they are lost in the hole of obscurity, because who gives a fuck if they aren't Black Legion or an aligned Warband.
Their only known event they appear in lore comes to us from Forge World, where the Reborn were said to have had a hand in the events leading to the Badab War: long before the whole debacle, the warband supported a chaos cultist uprising in a key system of the Badab sector. With the help of the Reborn, the cultists managed to turn the planet into an irradiated shithole Krieg-style. The entire planet became uninhabited and the trade routes in the system began collapsing, promting the High Lords to create the Malestorm Warders. The rest is history.